email   cat  class_price
0   email1@gmail.com  cat1            1
1   email2@gmail.com  cat2            2
2   email3@gmail.com  cat2            4
3   email1@gmail.com  cat2            4
4   email2@gmail.com  cat2            1
5   email3@gmail.com  cat1            3
6   email1@gmail.com  cat1            2
7   email2@gmail.com  cat2            1
8   email3@gmail.com  cat2            4
9   email1@gmail.com  cat2            2
10  email2@gmail.com  cat3            1
11  email3@gmail.com  cat1            1

test_df2 = test_df.groupby(['email','cat'])['class_price'].max()

email             cat
email1@gmail.com  cat1    2
                  cat2    4
email2@gmail.com  cat2    2
                  cat3    1
email3@gmail.com  cat1    3
                  cat2    4

email             cat      maxvalue
email1@gmail.com    cat2     2
email2@gmail.com    cat2     2
email3@gmail.com    cat3     3